I just got through camping this guy in that spot. Just logging in every 20 min, mollify the adds, and pull the placeholder. There were exactly 30 placeholders in a row before I saw Eldervine.

So he is Rare with a capital R.

A necro in zone says he also spawns in the gazebo right before the NW room where that map loc is. But I have never seen him there.

The actual fight is pretty easy. He hits you with the Bounce Knockback / DD right away, then a minute into the fight he does his slow AE. Remove Greater Curse fixes it. His HP are really low so if you have a full group you can kill him before he does it.